Particular embodiments generally relate to voltage level shift circuits.
Charge pumps are widely used in integrated circuits (ICs). These circuits may convert one direct current (DC) voltage level to another DC voltage level using switching capacitors that transfer a stored charge. FIG. 1 illustrates a prior art charge pump circuit 100.
Charge pump 100 includes switches 101-104, an inverter 106, a flying capacitor 107, and a reservoir capacitor 108. Inverter 106 provides a complimentary signal to switches 101-104 such that either switches 101-102 or switches 103-104 are closed at any one time. In a state that is not shown in FIG. 1, signal 105 activates (closes) switches 101-102 and de-activates (opens) switches 103-104 such that reference voltage Vin charges flying capacitor 107. As shown, signal 105 changes state (e.g. from a high to low state) and de-activates (opens) switches 101-102 and activates (closes) switches 103-104. In this state, flying capacitor is coupled to and charges reservoir capacitor 108. The terminals across reservoir capacitor 108 establish that Vout=−Vin.
Charge pump 100 generates a negative power supply rail thus allowing a circuit to use both the positive power supply rail (Vin) and the negative power supply rail (Vout). Although charge pump 100 provides a dual power supply, the circuit has limitations. For example, charge pump 100 has doubled the range of the voltage rails to +/−Vin from a range of ground to Vin. This may be a waste of power for some low voltage/low power applications that do not require this range of voltage. Also, since charge pump 100 generates Vout and another circuit (not shown) provides Vin, the current capability of the dual power supply may be uneven. In this case, loading of the power supplies may result in a lopsided power delivery and affect the operation of a circuit requiring symmetric power supplies.